A display screen magnifier is controllable by a user to magnify selected portions of a desktop including windows open on the screen. A magnified image is presented within a moveable magnifier frame. The magnified image corresponds to screen content located within a magnification region underlying the magnifier frame, magnified to a set level of magnification. The magnifier is configured to remain on top of the display screen desktop, including open windows, while it is activated. It also passes events of a user input device to windows located beneath the magnifier, and dynamically updates the magnified image to reflect changes in underlying screen content. The display screen magnifier is configured to operate this way through a leveraging of resources made available by application programming interfaces (APIs) of an operating system, namely a windows style, and a second stage screen content compositing engine associated with the windows style.